Ion pump



April 6, 1965 P. A. REDHEAD 3,176,907

ION PUMP Filed Aug. 25, 1962 2 SheetsSheet l I/VVENTOR PAUL A. RED/{FADPATENT AGE/VT April 6, 1965 P. A. REDHEAD ION PUMP 12 Sheets-Sheet 2Filed Aug. 25, 1962 lNVE/VTO PAUL A. REDHE D PATENT AGE/VT United StatesPatent 3,176,907 EON PUW Paul A. Redhead, Ottawa, Gntario, (Zanada,assignor to National Research Council, Ottawa, flutario, Canada, acorporation of (Ianada Filed Aug. 23, 1952, Ser. No. 218,917 8 Claims.($1. 230-69) This invention relates to a cold cathode ionization vacuumpump and more particularly to a magnetron type ionization vacuum pumphaving a discharge region in which electric and magnetic fields areproduced substantially at right angles.

The production of low pressures has been achieved in the past by the useof a combination of mechanical pumps and difiYusion pumps. In recentyears various types of ionization pumps have been developed. These pumpsdiffer from the mechanical and diffusion pumps in that they do notremove gas from the vacuum system but rather by immobilizing the gaswithin the system by a combination of chemical adsorption and entrappingof the ions of gas. Ionization pumps contain no fluid and therefore haveone salient advantage over diffusion pumps which use a fluid e.g. oil ormercury which can reach the vacuum system causing contamination Thereare two general types of ionization pumps known and in use at thepresent time. In the first type, which is known as a sputter-ion pump,the positive ions formed in the discharge region bombard a metalsurface, usually titanium and sputter metal atoms from this surface. Thesputtered material condenses on other portions of the device providedfor this purpose, entrapping and adsorbing the chemically active (otherthan inert) gases in the system. The second type of pump, which is knownas a getter-ion" pump contains a source of metal usually titanium, whichis evaporated. This evaporated material condenses on surfaces providedin the system, entrapping and absorbing the chemically active gases.

The sputter-ion pump does sufier from a serious detect which might becalled the historical effect, and which is one aspect of the generalre-emission effect. When a first gas A is fed into the system and pumpedaway, and then a second gas 13 is introduced, it has been found that gasB is pumped but a small amount of gas A is evolved from the pump. Thiseffect can be nullified in getter-ion pumps by covering the pumped gaswith a layer of evaporated metal to prevent the gas from beingre-emitted.

All ionization pumps using a magnetic field in use at the present timeare of the parallel-field type in that the electric and magnetic fieldsare parallel. Because of the geometry of parallel-field pump the magnetsrequired to produce the magnetic field are large and heavy.

It is an object of the present invention to provide an ionization vacuumpump which is light in weight, simple in design, and has greatlyincreased pumping speeds in relation to its size and weight.

It is another object of the invention to provide an ionization pump thathas its pumping area separated from the discharge region allowing wideflexibility in the d sign of the pumping region.

Another object of this invention is to provide an ionization pump inwhich the magnetic field structure forms part of or is in closeproximity to the envelope thus achieving a lighter and more eificientdevice.

' Another object of this invention is to provide a pump whose geometryis such that a tube of large diameter can be readily used to connect thepump to the system required to be evacuated, allowing high pumpingspeed.

These and other objects of the invention are realized by providing anionization pump comprising a gas-tight enclosure defining twointerconnected volumes, the first 3,l?fi,d? Fatented Apr. 8, 1965 saidvolume defining a discharge region and the second said volume defining apumping region, means for connecting the pumping region to a system thatis to be evacuated, a metallic structure mounted in the pumping regionto provide metallic atoms for the absorbing of the gas molecules beingpumped, a magnetic circuit forming part of said enclosure and havingpoles forming a magnet gap region therebetween, said magnet gap regionbeing said discharge region, a permanent magnet in said magnetic circuitto produce a strong magnetic field in the discharge region, a cathodepositioned adjacent the interconnection between the pumping region andthe discharge region, an anode positioned adjacent the magnet gap andopposite the said cathode, and lead means to supply a potential to theanode to produce an electric field in the discharge region, saidelectric field being transverse to the said magnetic field.

In drawings which illustrate embodiments of the invention,

FIGURE 1 is a cross-sectional view of a getter-ion pump according to theinvention,

FIGURE 2 shows section AA of FIGURE 1,

FIGURE 3 is a three-quarter view of a getter-ion pump according to theinvention showing the positioning of the permanent magnets,

FIGURE 4 is a plural version of the pump capable of increased pumpingspeeds.

Referring to the drawings, FIGURE 1 shows a getterion pump according tothe invention having an envelope indicated generally as 1, formed by anupper pole piece '7 and a lower pole piece 8, a generally cylindricalshaped outer barrier wall 13 made of stainless steel or othernonmagnetic material, a glass-to-metal seal 6 and tubular member 11which would be connected to the system to be evacuated. This envelopedefines a first volume shown generally as 2, to be referred to below asthe pumping region. Pole pieces '7 and 8 have raised portions extendingtowards each other, forming north and south magnetic poles N and S anddefining a second region (magnet gap) between them shown generally as 3.The poles should be relatively close to each other so that a strongmagnetic field can be produced in the magnet gap. This requirementhowever must be consistent with the need to provide space for adischarge of sutficient size to he formed in the gap. The faces of themagnetic poles are covered with thin sheets 10 of a metal preferablytitanium. Permanent magnets 14 which are magnetized along their lengthare positioned to contact pole pieces 7 and S which are preferably madeof low reluctance mild steel to form a magnetic circuit and provide astrong magnetic field in magnet gap 3. Cylindrical tubes 9 of sheettitanium are positioned adjacent the interconnection between pumpingregion 2 and magnet gap 3 and form, with sheets 19, a cathode. An anodein the form of a cylindrical ring 12 is positioned outwardly of magnetgap 3. The cathode 9 and 10 and pole pieces 7 and 8 are operated atground potential and anode 12 is operated at high positive potentialwith the result that an electric field is set up in the magnet gapregion 3 generally at right-angles to the magnetic field mentionedabove. An evaporator 26 is mounted on rod 5 having a heater winding 21through which current can be passed by means of lead 22 which passesthrough glass seal 6. An accelerator electrode 23 in the form of twocylindrical tubes is positioned inwardly of cathode tubes 9. Rods 24passing through the glass seal 6 hold the accelerator electrodes inposition and provide an electrical connection so that the acceleratorelectrodes can be operated at a negative potential with respect to thecathode. A typical evaporator would comprise a sapphire rod on which iswound a tungsten heater wire, the heater wire in turn having a windingof titanium metal wound over its piece 7' by several bolts 31. A. goldwire O ring seal 33' is compressed by the tightening of bolts 31providing the required seal between pole piece 7 and insert ring 32..

Barrier wall 13 is welded or brazed to lower pole piece Cylindricalmember 11 is connected by means of a glass=to.-metal' seal to thetubulation 19 of the external system. The gl'ass-to-metal seals can bestandard Kovarglass seals or ceramic-metal seals.

FIGURE 2. is a cross-sectional view A-A through the pump shown in FIGURE1 and shows a method of mounting the permanent magnets 1-4. Althoughtwelve magnets are shown in this figure the number could be varied.quite. widely. If pole pieces 7 and 8 shown in FIGURE 1, wereconstructed. of permanent magnet material a more efiicient device wouldbe realized with the result that smaller or fewer permanent magnetswould .be required. From this figure it will be seen that the device asshown, in FIG. 1 is, with the exception of the magnets, a figure ofrotation and that. the discharge region (mag-net gap region) is anannulus completely encircling, the pumping region. The discharge regionwhich acts as a source of positive ions is able to supply ions toconverge on the sputter cathode from a 360 sector. Itv can be seen thatthis results in a very efiicient device.

7 In operation a discharge is established in the region bounded by thepole faces, the anode and the catode. This, discharge region coincidesgenerally with the magnet gap region 3 mentioned above. Stray electronsin the discharge region are attracted towards the anode which is at a.high positive potential with reference to the cathode. During theirflight they strike gas molecules and if the electrons have sufficientenergy they will ionize the gas moleculesand' produce electrons andpositive'ions. These positive ions are attracted towards the cathode anda large proportion will have sufiicient energy to pass through slit 16,in the cathode tubes into the pumping region. In the; pumping region thepositive ions emerging from the Slit are accelerated by a'negativepotential on the accel'erator electrodes 23. The evaporator 20 isoperated at a positive potential so that no positive ions can reachit.The positive ions move radially inward, turn around, and finally strikethe accelerator electrode. A heating current passed through windings 21-evaporates titanium from the surface of the evaporator. This evaporatedtitanium condenses onto the surface of the accelerator electrodeentrapping positive ions and absorbing molecules. of the gas present.This gives rise to pumping action. The pumping region. and the dischargeregion overlap to some extent with the. discharge extending somedistance into the pumping region and pumping action taking place in thedischarge region. It should be pointed out, that the action described.here as absorption also includes. the phenomenon usually described asadsorption,

The crossed electric. and magnetic fields produce a discharge regionsimilar to that in a magnetron. Electrons on their way toward the anodetravel in spiral orbits .which increases greatly the probability oftheir striking and ionizing a. gas molecule. This results in highlyincreased efliciency.

As mentioned above, the getter-ion type of pump has the advantage thattitanium metal can be evaporated onto theaccelerator electrodes, beforepumping a new gas, in sufiicient quantity to cover up any atoms of a gaspumped during a previous operation. This eliminates the: historical.eifect" mentioned above.

Typical operating levels of voltage and current in a getter-ion pump ofthe type described above would be, for example, +2 to +7 kv. on theanode, 2 kv. on the accelerator electrodes, the evaporator slightlypositive i or at ground or floating, and. 7 to. 8 amperes through. theheating windings. The magnetic flux density in the gap would beapproximately 1000 gausses. These values, of course, can be varied quitewidely.

FIGURE 3 is a three-quarter view of a version of the pump showingexternal features especially the method ofture of these pumps could varyconsiderably within the scope of the invention. For example, the upperand lower pole pieces and the permanent magnets could be, made as aunitary structure out of magnetic material. The stainless steelbarrierwall could be in the form of a thin sheet in contact with a portion ofthe inner surface of this unitary structure. The anode structurepositioning rod' could be taken through both the stainless steel sheetand the magnetic material by means of a glass-to-metal' seal. In somecases, the barrier wall might be eliminated altogether. In ionizationpumps, titanium has been the metal most generally used to. absorb andentrap the gas molecules. Other suitable metals such as molybdenum,tantalum, tungsten, zirconium, iron, calcium, barium, etc., might beused. The present invention is not primarily concerned with the metalused.

What is claimed is:

1. An ion pump comprising a cathode and an anode spaced apart to definetherebetween a glow-discharge zone in which gas ionization isaccomplished, electrical connections to said cathode and anode whichwhen con-.

nected to an external electrical potential source willcause I anelectrical field to be established between the anode and cathode, amagnetic circuit arranged to established a magnetic field transverse tothe said electrical field, the glowdischarge zone having a longdimension transverse to the magnetic field which is at least; severaltimes greater than that dimension of the zone which is parallel to. themag; netic field, an envelope defining a sorption zone in which ions andmolecules are sorbed, the cathode extending along said long dimension,the cathode serving to separate the glow-discharge and sorption zones,the cathode being posir tioned to permit ready escape of positive ionsfrom the glow-discharge zone i-ntothe sorption zone, a source ofsorption metal positioned within said sorption zone, means defining asubstantial sorption area in the sorption zone on which sorption metalis deposited, the sorption area being outside of the glow-dischargezone, and being, substantially greater than the area of cathodeseparating the glowdischarge zone from the sorption zone, and means toconnect the said sorption zone to a structure to be evacuated. 2. An ion:pump comprising an anode and perforated cathode connected toa source ofelectrical potential so as to establish an electrical field betweenthem, a magnetic circuit arranged to establish a magnetic field crossingthe said electrical field generally at right angles, said crossedelectrical and magnetic fields defining anannular glowdischargeionization zone whose inner boundary is defined by said perforatedcathode, an envelope defining a sorption'zone whose outer boundary isdefined by the perforated cathode, a. source of sorption metal vaporspositioned within the sorption zone, a substantial sorption area withinthe sorption zone on which sorpt-ionmetal vapors are condensed, thesorption area being outside of the glow-discharge zone, and means toconnect said sorption zone to a structure to be evacuated. V 3. An ionpump as in claim 1 in which the source of sorption metal is anevaporator formed of a body sava e? of sorption metal and a heaterwinding to evaporate said sorption metal.

4. An ionization pump as in claim 2 in which said perforated cathode isdefined by a plurality of cathode plates defining spaces therebetween.

5. An ionization pump as in claim 2 in which said perforated cathode isdefined by a pair of cathode rings, said rings being coaxial andlongitudinally spaced to define a gas conductance path between saidionization and sorption regions.

6. A cold cathode ionization pdmp comprising a gas tight enclosuredefining two interconnected volumes, the first said volume being adischarge region and the second said volume being a pumping region,means for connecting the pumping region to a system that is to beevacuated, a metallic structure mounted in the pumping region to providemetallic atoms for the absorbing of the gas molecules eing pumped, amagnetic circuit forming :part of said enclosure and having polesforming a magnet gap region therebetween, said magnet gap region beingsaid dis charge region, a permanent magnet in said magnetic circuit toproduce a'magnetic field in the discharge region, a cathode having asurface capable of absorbing gas molecules positioned adjacent theinterconnection between the pumping region and the discharge region, ananode positioned adjacent the magnet gap region and opposite the saidcathode, and lead means to supply a potential to the anode to produce anelectrical field in the discharge region, said electric field beingtransverse to the said mag netic field.

7. A cold cathode ionization pump of the ion-getter type comprising agas tight enclosure defining two interconnected volumes, the first saidvolume being a discharge region and the second said volume being apumping region, means for connecting the pumping region to a system thatis to be evacuated, a metal evaporator formed of a source of metal and aheater winding to evaporate said source of metal mounted in the pumpingregion to provide metallic atoms for the absorbing of the gas moleculesbeing pumped, a magnetic circuit forming part of said enclosure andhaving poles forming a magnet gap region therebetween, said magnet gapregion being said discharge region, a permanent magnet in said magneticcircuit to produce a magnetic field in the discharge region, a cathodehaving a surface capable of absorbing gas molecules posi- No referencescited.

LAURENCE V. EFNER, Primary Examiner.

WARREN E. COLEMAN, Examiner.

1. AN ION PUMP COMPRISING A CATHODE AND AN ANODE SPACED APART TO DEFINETHEREBETWEEN A GLOW-DISCHARGE ZONE IN WHICH GAS IONIZATION ISACCOMPLISHED, ELECTRICAL CONNECTIONS TO SAID CATHODE AND ANODE WHICHWHEN CONNECTED TO AN EXTERNAL ELECTRICAL POTENTIAL SOURCE WILL CAUSE ANELECTRICAL FIELD TO BE ESTABLISHED BETWEEN THE ANODE AND CATHODE, AMAGNETIC CIRCUIT ARRANGED TO ESTABLISHED A MAGNETIC FIELD TRANSVERSE TOTHE SAID ELECTRICAL FIELD, THE GLOWDISCHARGE ZONE HAVING A LONGDIMENSION TRANSVERSE TO THE MAGNETIC FIELD WHICH IS AT LEAST SEVERALTIMES GREATER THAN THAT DIMENSION OF THE ZONE WHICH IS PARALLEL TO THEMAGNETIC FIELD, AN ENVELOPE DEFINING A SORPTION ZONE IN WHICH IONS ANDMOLECULES ARE SORBED, THE CATHODE EXTENDING ALONG